1. Technical Field
The present invention relates to a gas specie and aerosolized-gas mixture particle separation device and process, more generally described as a separation device to separate a gas mixture, or a gas with aerosolized mixture into its heavy and light components by mass differences through a combination of supersonic expansion, core flow collimation, and differential diffusion from a pressure gradient in a centrifugal field.
2. Background Art
Key energy and chemical industries currently require advanced gas separation technologies to be developed so that efficient production of critical products can be realized. Whereas accepted separation technology is now used or is available, capital or energy expense is prohibitive so that acceptable production is not viable. Generally the requirements of these industries is for high volume, possibly with high pressure and elevated temperature processing of mixed gas flows to extract contaminants to yield a purified product gas.
It has been known for many years that gas centrifugation may be used to separate two-component gas mixtures, particularly for uranium isotope enrichment. In this application, a gas mixture is fed into a complex rotor assembly that spins at high speed inside of an evacuated casing. Because the rotor spins extremely rapidly, centrifugal force results in the gas occupying only a thin layer next to the rotor wall, with the gas moving at approximately the speed of the rotor wall. The centrifugal force has a property similar to gravity in that it accelerates molecules or particles at a rate which is independent of their masses. It follows that centrifugal force by itself cannot be used to separate molecular species. All it can do is create a pressure variation in the gas, against which molecules or particles of different masses diffuse at different rates. It is this differential diffusion against a pressure gradient that underlies the centrifuge method. The separation factor is solely the difference of the molecular weights (ΔM) between species in the gas mixture, allowing diffusion concentration of heavy components along the moving wall and the light component separation diffusion away from the moving wall.
The use of the centrifuge method for isotope separation was first suggested in 1919, but efforts in this direction were unsuccessful until 1934, when J.W. Beams and co-workers at the University of Virginia applied a vacuum ultracentrifuge to the separation of chlorine isotopes. Although the U.S. abandoned the centrifuge method during the Manhattan Project, the gas centrifuge uranium enrichment process has been fully developed by others and is used to produce high-enriched uranium (HEU) and low-enriched uranium (LEU). These classical centrifuge designs are now used in variety of different separation processes.
The main subsystems of these classical centrifuge methods are: (1) rotor and end caps, (2) top and bottom bearing system, (3) electric motor and frequency controller, (4) center post, scoops and baffles, (5) vacuum system, and (6) casing. Due to the high rotation speed of the rotor, the rotor must be formed of materials selected to reduce the risk of bursting, and the bearing system must be capable of the high rotation speed necessary for separation. The separative capacity of a single centrifuge increases with the length of the rotor and the rotor wall speed. Consequently, centrifuges having long and high speed rotors are desirable. Although the separation factors obtainable from the centrifuge method are large compared to gaseous diffusion, several cascade stages are still required to produce even LEU material. Furthermore, the throughput of a single centrifuge is usually small, which leads to rather small separative capacities for typical centrifuges. Although significant effort has been expended to develop traditional centrifuge based separation methods, there are a number of limitations to be solved, including for example:
1. It is necessary to establish a circular flow pattern within the centrifuge to recirculate the gas mixture because the separation factor of the centrifuge is determined by the ratio of isotopic abundances at the axis of rotation and at the centrifuge wall.
2. Due to the extremely high speeds necessary to induce meaningful separation, the centrifuge must be precisely machined and balanced and in most cases requires operation in a vacuum system to reduce aerodynamic drag and use of magnetic bearings to reduce friction and vibrations. Further, the high rotational speeds results in significant mechanical stresses on the centrifuge wall requiring the use of exotic materials and fabrication techniques to avoid rupture.
4. The pressure differential between the center of the rotor and the outer wall drives the diffusion rate and thus the theoretical efficiency of the centrifuge. However, it also defines the pressure applied to the gas at the outer wall of the centrifuge and if improperly constructed, the large induced pressure may cause the gas to sublimate and condense on the wall of the centrifuge. To avoid this, the center of the centrifuge must be held at a vacuum, significantly limiting the overall throughput of the centrifuge.
An alternative separation process using aerodynamic techniques in conjunction with a specified nozzle geometry was developed E.W. Becker at the Karlsruhe Nuclear Research Center in Germany, separation nozzles of this type are generally referred to as a Becker process nozzle. This process depends upon diffusion driven by pressure gradient effect, similar to the gas centrifuge. In effect, aerodynamic processes can be considered as nonrotating centrifuges. In many cases the centrifugal forces, as used in the Becker process, must be increased by including a cut gas or separation enhancing gas that allows the gas to accelerate faster. For example in the case of uranium separation, this is usually achieved by the dilution of the uranium hexafluoride (UF6) with a carrier gas, (hydrogen or helium), that allows the gas to achieve a much higher flow velocity for the gas to be separated. In this process, the gas mixture of the separation gas and the carrier gas is compressed and then directed along a curved wall at high velocity through a convergent-divergent nozzle. The heavier molecules move preferentially out toward the wall relative to those containing the lighter molecules. At the end of the deflection, the overall gas jet is split by a skimmer into a light enriched fraction and a light depleted fraction, which are withdrawn separately.
Generally the curved nozzle wall of the nozzle may have a radius of curvature as small as 0.0004 inch. Production of these tiny nozzles by manufacturing is technically demanding, and the overall process typically includes stages having multiple vessels containing hundreds of separation elements, gas distribution manifolds, gas coolers to remove the heat of compression, and centrifugal compressors to pressurize the flow.
The Becker process as adapted for separation of uranium, uses a jet of gas consisting of roughly 95 percent hydrogen and 4 percent uranium gas that is expanded through a narrow slit nozzle. The gas moves at high speeds, parallel to a semicircular wall of very small radius resulting in a gas speed comparable to that at the periphery of a gas centrifuge. If the speed of the gas is 400 m/s, and the radius of curvature is 0.1 mm, then the centrifugal acceleration achieved is about 160 million times gravity. These accelerations exceed even those in high speed centrifuges. Additionally these accelerations are achieved with no moving parts. In this method, the centrifugal forces on the molecules cause the streamlines of the heavier components of the gas to move closer to the curved wall compared to those of the lighter components as the gas flows around the semicircle (A streamline in a flowing gas is a line across which no net material transport takes place). At the other side of the semicircular wall, where the gas has changed direction by 180°, a sharp skimmer separates the flow into an inner light fraction and an outer heavy fraction.
Diffusion across a streamline, used by a Becker process nozzle, is analogous to the diffusion against a gravitational or centrifugal force. So all separation processes in the gas occur in directions perpendicular to the streamlines because no net material transport takes place across the streamline. As an example, in a uranium centrifuge the gas is moving in circular paths, so the streamlines are concentric circles. The isotope separation takes place in the radial direction, perpendicular to these streamlines. If a streamline is curved, this implies that the gas is being accelerated, and that a pressure gradient or force must exist perpendicular to the streamline.
In many instances a cut gas is necessary to accelerate the flow through a Becker nozzle. For example in the case of UF6 separation, a significant quantity of hydrogen is typically used to reduce the average molecular weight of the process gas to enable high speed flow to maximize separation effects and it also provides a drag force that enhances the separation process. In many cases the cut gas must dominate the flow, thereby reducing the overall separation efficiency and throughput of the system. For example, in the case of UF6, the hydrogen cut gas (H2) is typically in excess of 95% of the volume of the process gas passing through the Becker process nozzle. The use of a cut gas coupled with the very small size of the Becker nozzles causes the rate of material flow through the process to be small in each nozzle, and several thousand nozzles are necessary to make an enrichment stage of any reasonable size.
As a result of these requirements, the Becker process has a number of significant limitations that have reduced its applicability to general purpose enrichment including:
1. The small size of the curvature wall requires that the nozzle and skimmer components are also minute in size, requiring the components be made of foil material and bonded to assemble even one nozzle. Nano-fabrication is required to form the nozzles, with specifications to the 0.001 mm tolerance.
2. The operating pressure of the process gas is at several bar, usually below 6 bar. The addition of specialized compressors in the cascade add energy to the operating costs to maintain the process at separation pressure.
3. The flow stream along the curvature wall has little if any centrifugal force acting on that streamline, so the 95/5 mixture along the curvature wall dilutes the concentrating heavy isotope stream at extraction, thereby reducing the separation factor by dilution.
4. To reduce the cost of the processing, the add-in hydrogen gas must be cleaned at the end of the process for reflux back into the system, adding another separation process to the cascade.
Another alternative separation technique disclosed in the prior art is the use of a Pitot probe in a supersonic gas mixture, first explained by Fenn and Reis (1963), of the type exemplified in the following U.S. Patents—U.S. Pat. No. 3,465,500 to Fenn (1969) and U.S. Pat. No. 3,616,596 to Campargue (1971). It was disclosed that at suitably low Reynolds numbers in a free-expansion jet of nitrogen/hydrogen gas mixture, the gas entering the probe was enriched in the heavier of the two species due to the stagnation effects at the probe tip. It had previously been considered that gas samples taken by such probe effect contained the two-molecular composition of the free-expansion jet that had been separated by radial diffusion effects in the jet upstream of the probe. However, in later experimentation by Rothe (1966), it was revealed that the magnitude of the probe-induced separation measured by Fenn and Reis was up to 50 times greater than that due to radial diffusion in the free jet alone. This determination revealed that it was the shock front preceding the probe that caused lighter specie to follow streamlines around the probe, while heavier specie passed into the probe inlet. This effect has been experimentally and theoretically tested for the separation effect causing minor separation of isotopes, primarily for the potential use in uranium isotope enrichment. Due to the small degree of separation caused by a single probe, the method has not progressed to any commercial degree.
M. R. Bloom, the present inventor, in the early 1990's developed an integrated aerodynamic device and the centrifuge method for the separation of lower molar weight gas mixtures commonly found in the energy and chemical industry sectors. The mechanical device of his work is described in U.S. Pat. No. 5,902,224 to Bloom (the '224 Patent), for a device used for gas mixture separation. In the '224 Patent, a centrifuge device is described for the separation of components of low-molar weight gas mixtures, including natural gas, air, and contaminated air. In this centrifuge device construction, a narrow-gap centrifuge is built consisting of many individual and stacked plates that are spaced from one another to form the centrifuge rotor, and the central area at the axis of rotation for this centrifuge is an open expansion chamber that extends through the height of the centrifuge. The '224 Patent discloses a narrow gap centrifuge with a stationary housing, a rotor with multiple, stacked, inverted pyramidal plates that form channels between the plates. As lighter constituents are separated from the gas mixture they travel down through the device for extraction.
Although the embodiments of the present device and method are described in combination with and in some cases contrasted to the theory and method of this previous art, this is not intended to limit the claimed invention in any sense.